Field of the Disclosure
The present disclosure generally relates to vehicle air suspensions. More particularly, the present disclosure relates to control systems for the air suspensions of 6×2 vehicles.
Description of Related Art
In North America, the majority of semi tractor-trailer combinations with tandem rear axles run in what is known as a 6×4 configuration. This nomenclature designates that six independent wheel positions exist on the tractor (i.e., two for the front axle and two for each of the two rear axles) and that four of those wheel positions are driven to provide propulsion of the vehicle. Typically, this is intended to mean that the four rear wheel positions are driven, with the two front axle wheel positions being non-driven and having the primary purpose of steering the vehicle. Additional configurations exist such as 6×2 (only one of the two rear axles is driven) and 4×2 (a truck with two axles having only one driven axle) configurations, to a lesser degree. The 4×2 configuration is limited by federal bridge laws to only allow a certain total vehicle load, whereas the 6×2 configuration has equal load carrying capability to a 6×4 configuration.
The key advantage of a 6×4 configuration is traction, while the key drawback is reduced fuel economy due to increased parasitic losses from the additional gearset needed to drive the second axle. When fuel prices are low, fleets tend to specify the 6×4 configuration to improve productivity, as fewer trucks will get stuck due to traction problems, especially in inclement weather such as snow and ice. As fuel prices have increased significantly in recent years, fleets are turning to 6×2 configurations as a possible method of improving their overall operating cost of fuel.
6×2 configurations have been widely used in Europe and other parts of the world for many years. To overcome the inherent traction issues, European tractors are equipped with Electronically Controlled Air Suspensions (ECAS), which incorporate a control unit, pressure sensors, wheel speed sensors, and valves with certain control logic to shift load toward the driven axle and away from the non-driven axle when wheel-slip is detected. By increasing load on the driven axle, more tractive effort is attained and the vehicle can move at slow speeds until is able to regain enough traction, at which point the system reverts to its normal operation maintaining a 50/50 load bias between both axles of the tandem. ECAS systems are a standard offering in Europe, however, they have not been used widely in North America due to their added cost and complexity. In North America, when 6×2 configurations have been utilized, a manual “air dump” valve has typically been run to the cab, giving the driver the option to release air from the non-driven axle's suspension when needed, thus transferring load to the driven axle. While simple and low cost, this method allows for operator error, which could inadvertently leave excessive load on the driven axle violating federal bridge laws. In order to be used effectively, this would require additional training and experience for the driver.
Systems have begun to be marketed in North America for a control module for 6×2 configurations that would automate this process, similar to an ECAS system. In all of these cases, the system monitors wheel speed and direction and, upon wheel slip, adjusts air pressure to put more load on the driven axle. Again, once traction is regained and the vehicle returns to normal speeds, the load bias is returned to 50/50 between both axles of the tandem.
Those skilled in the art of tire design, use and wear, will recognize an inherent design short-coming of a 6×2 tractor with 50/50 load bias on the tandem. A key difference between a 6×4 configuration and a 6×2 configuration is that a 6×4 configuration splits the torque transferred from the engine between two driven axles. In the case of a 6×2 configuration, 100% of the drive torque must go through the single driven axle. This increased torque at the same axle load will proportionally increase the longitudinal slip experienced by the tire. Longitudinal slip is a phenomenon that all rubber pneumatic tires experience when rotating to drive a vehicle. It is a slip that occurs in the direction of travel. This slip causes wear of the tire over time and, with the increased torque applied in a 6×2 configuration, will significantly reduce the tire life that fleets experience.
If all vehicles were to drive around fully loaded, increased longitudinal slip would likely not be a significant issue, as the increased wear of the driven axle tires might be offset by the improved wear of the non-driven axle. Rarely, however, is this the case. According to the most recent study by the US Department of Transportation, 53% of loads carried by tractor trailer combination class 8 vehicles are below 60,000 pounds. Assuming the front steer axle maintains a load of approximately 12,000 pounds, and equal distribution of the remaining load between the other axles, that would put approximately 24,000 pounds each on the tractor and trailer tandems. With 50/50 load bias, the driven axle would be loaded to only around 12,000 pounds of its legal 20,000 pound limit while still taking 100% of the drive torque.
In view of the foregoing shortcomings of known air suspension control systems, it would be advantageous to provide an air suspension control system that improves traction and reduces tire wear when the associated vehicle is under-loaded.